I've just today fixed an icomplete download with the par2 utility. (apt-get install par2 - on debian).
The .nzb files often (always?) contain parity files, which then with:
par2repair file.par2
recreates the missing stuff... COOL!
I found that just trying to download the same thing again actually didn't miss anything the second time, so I'm not sure a pre-check would be foolproof anyway...